The present invention relates to a driving mechanism for optical parts, especially an objective of a pickup in an optical information reader which reads information contained in a disc-shaped information recording medium.
In conventional optical information readers of a video disc player, D.A.D. (Digital Audio Disc) player or the like, a driving mechanism for a pickup including an objective is constructed as shown in FIG. 1, for example.
A pickup D in which an objective B is disposed in a holding cylinder C in order to condense light from a light source or reflected light from a disc A to be focused is supported on a mount frame F by a damper E in such a manner as to reciprocate. As a driving mechanism G for moving the holding cylinder C in the focus direction perpendicularly to the disc surface A in order to adjust the focus of the above objective B to the disc A, a driving coil I wound round a bobbin H mounted on the lower outer periphery of the holding cylinder C is inserted in a magnetic gap K of a magnetic circuit g for the focus direction drive, and as a driving mechanism J for moving the holding cylinder C in the tracking direction along the radius of the disc A, driving coils L are respectively wound round two bobbins M, M in the opposite phase, which are disposed at the symmetrical positions of the outer periphery of the holding cylinder C, and magnets O secured to each inside of the described mount frames F are respectively inserted in the bobbins M, M. In case of driving the objective B in the focus direction, the driving coil I is energized so that the holding cylinder C is driven by the magnetic interaction with the magnetic circuit g. Thus, the focus of the objective B is adjusted. In case of driving the objective B in the tracking direction, the driving coils L, L are energized for the tracking direction drive so that the holding cylinder C is driven by the magnetic interaction with the magnets O.
In case of reproducing a signal recorded on a disc by the thus constructed driving mechanisms, the pickup has to be moved along the radius of the disc A by movement of a slide base (not shown). The driving mechanism portion for the pickup D, however, is large-sized as described above and the pickup D cannot approach sufficiently the axis of a motor P so that it is impossible to use a large-sized motor P with a large output for reproduction. Accordingly, the driving torque of the motor P is decreased so that the rise time of an apparatus is lengthened, and the response in its initial operation gets worse. These problems are serious in case of using a small-diameter compact disc as an information recording medium.